How to tell Entity Framework that my ID column is auto-incremented (AspNet Core 2.0 + PostgreSQL)?

问题

Code is simple. Tag.cs entity:

public partial class Tag
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
}

HomeController.cs class:

public async Task<IActionResult> Index()
{
   tagRepository.Insert(new Tag 
               { 
                   Name = "name", 
                   Description = "description" 
               });
   await UnitOfWork.SaveChangesAsync();  // calls dbContext.SaveChangesAsync()

   return View();
}

TagRepository.cs class:

    // Context it's a usual DBContext injected via Repository's constructor
    public virtual void Insert(TEntity item)
                => Context.Entry(item).State = EntityState.Added;

Tag table was created by running:

CREATE TABLE Tag (
    ID SERIAL PRIMARY KEY,
    Name text NOT NULL,
    Description text NULL
);

When run my application I get an error:

fail: Microsoft.EntityFrameworkCore.Database.Command[20102]
      Failed executing DbCommand (28ms) [Parameters=[@p0='?', @p1='?', @p2='?'], CommandType='Text', CommandTimeout='30']
      INSERT INTO "tag" ("id", "description", "name")
      VALUES (@p0, @p1, @p2);
Npgsql.PostgresException (0x80004005): 23505: duplicate key value violates unique constraint "tag_pkey"
   at Npgsql.NpgsqlConnector.<DoReadMessage>d__157.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Runtime.CompilerServices.ValueTaskAwaiter`1.GetResult()
   at Npgsql.NpgsqlConnector.<ReadMessage>d__156.MoveNext()

As you can see Entity Framework tries to send id=0 value to the DB, but there is already a record with id=0 in my DB and that's why it throws duplicate key error.

I didn't find any answer so far and my question is: how can I get rid of this error and tell Entity Framework that my id column is auto-incremented and there is no need to update it?

回答1:

You have to use here "ValueGenerationOnAdd()". As the issue you are getting is already reported on GitHub. Please find the below link.

https://github.com/npgsql/Npgsql.EntityFrameworkCore.PostgreSQL/issues/73

You can find more info regarding Generated Value pattern from following link.

Value generated on add

public classs SampleContext:DBContext{
public DbSet<Tag> Tag { get; set; }

protected override void OnModelCreating(ModelBuilder modelBuilder){
    modelBuilder.Entity<Tag>()
        .Property(p => p.ID)
        .ValueGeneratedOnAdd();
 }
public class Tag{
  public int Id { get; set; }
  public string Name { get; set; }
  public string Description{get;set;}
  }
}

Source:- https://www.learnentityframeworkcore.com/configuration/fluent-api/valuegeneratedonadd-method

Hope this will help

回答2:

According to the Postgre SQL docs here is an example which shows how to achieve the desired result:

protected override void OnModelCreating(ModelBuilder modelBuilder)
   => modelBuilder.Entity<Blog>().Property(b => b.Id).UseIdentityAlwaysColumn();

回答3:

After a lot of wasted time on research I've solved it absolutely unexpected. Actually, the solution to my problem lies on the surface, because the problem is not in EF and not in it's stuff. In my case it was SEQUENCE. I mean, involved table has serial column and related sequence just was changed by some side-effect. In another words - sequence was restarted from 1 and at the same time the table is already having values with 1, 2, 3 ... etc. That's why postgres throws 'duplicate key' error. So, the solution is:

ALTER SEQUENCE "your_table_seq" RESTART WITH your_value;

来源：https://stackoverflow.com/questions/50202735/how-to-tell-entity-framework-that-my-id-column-is-auto-incremented-aspnet-core

标签

asp.net-core

entity-framework-core

auto-increment

npgsql